Active ray-curable compositions that are cured by active energy rays such as ultraviolet radiation or electron beam have been hitherto put to practical use in various applications such as coating materials for plastics, paper, wood, inorganic materials and the like, adhesives, printing inks, printing circuit boards and electrical insulation relationships.
Furthermore, regarding inkjet ink systems which use these polymerizable compositions, there are available ultraviolet-curable inkjet inks that are cured by ultraviolet radiation (hereinafter, appropriately abbreviated to “UV”). Inkjet systems that use these ultraviolet-curable inks have fast-drying properties, so that recording can be achieved on recording materials which lack ink absorbing properties, and therefore, attention has been paid to these inkjet systems in recent years.
However, in an image forming method using these ultraviolet-curable inkjet systems, there is a problem that adjoining dots coalesce at the time of high speed recording for example, in the case of a line recording system, conveyance speed of the recording material: 30 m/s or higher, and in the case of a serial (shuttle) recording system, printing speed: 50 m2/hr or higher), and the image quality deteriorates.
As a technology which is capable of inkjet recording regardless of the kind of the recording material and prevents “coalescence of adjoining dots” that becomes a problem at the time of inkjet recording, it is known to use a UV-curable ink containing a gelling agent (see Patent Literatures (hereinafter, abbreviated as PTL) 1 to 3).
For example, PTL 1 discloses an inkjet ink containing an oil to gelling agent and a photopolymerizable compound, and cationic polymerizable compounds or radical polymerizable compounds are disclosed as the photopolymerizable compounds.
Furthermore, PTL 2 discloses a phase change ink containing a curable monomer, a gelling agent, a branched polymer having a photoinitiator moiety in the molecule, and a coloring material, and radical polymerizable compounds are disclosed as the curable monomer.
PTL 3 discloses a phase change ink containing a white pigment, a curable monomer, a gelling agent and a photoinitiator, and radical polymerizable compounds are disclosed as the curable monomer.
However, in these ink compositions disclosed in PTL 1 to 3, control of dot coalescence at the time of high speed recording such as described above is still insufficient. Furthermore, there is a problem that ejection stability is not sufficient, and high quality images cannot be formed with satisfactory reproducibility.
Furthermore, in general UV-curable inks, the problem that printed matters curl up as a result of curing shrinkage at the time of UV irradiation, or that when a printed matter is bent, the image film is damaged, is prevented by using the relative low molecular weight, monofunctional and bifunctional radical polymerizable compounds disclosed in PTL 1 to 3 as the polymerizable compounds. However, in inks containing a gelling agent and a photopolymerizable compound, when a relatively low molecular weight radical polymerizable compound having a molecular weight of less than 300 is used, there is a problem that the stability of the ink composition at a temperature capable of ink ejection becomes insufficient, or there is a problem in terms of environmental hygiene that the foul odor of the ink itself or the foul odor of the printed matter after wring, is strong.
As a technology of suppressing curing shrinkage without impairing curability, it is well known to use the trifunctional or higher functional polymerizable compound having an ethylene oxide (EU) unit in the molecule as disclosed in PTL 1. However, when a polyfunctional polymerizable compound of trifunctionality or higher functionality having many ethylene oxide (EU) units in the molecule is used in a large amount, hydrophilicity of the ink as a whole becomes very high. Therefore, it is difficult to allow a gelling agent to exist stably in a highly hydrophilic ink, and an ultraviolet-curable inkjet ink composition which satisfies all of high image quality, curability, curing, shrinkage has not been known,